Sputtering, alternatively called physical vapor deposition (PVD), is used to deposit several different layers of metals and related materials in the fabrication of semiconductor integrated circuits. In two related and demanding applications, first a thin barrier layer is sputtered onto the walls and bottom of a narrow hole etched into and often through an inter-level dielectric layer, most typically composed of silicon oxide or similar oxide materials. The barrier layer prevents the migration of metal into the oxide and oxygen into the metal filling the via hole. For copper metallization, the barrier layer is often composed of tantalum, for example, a bilayer of Ta/TaN. Then a thin seed layer of copper is deposited over the barrier layer to serve both as a seed layer and an electroplating electrode for the subsequent filling of copper into the hole The remainder of the hole is then filled with a metal to serve as an electrical connection either vertically in a via penetrating the dielectric layer or horizontally in a trench interconnect formed in the surface of the dielectric layer. The barrier layer prevents diffusion between the metal and the oxide dielectric and thereby prevents oxygen from degrading the metal conductivity and metal from decreasing the resistivity of the dielectric.
Both the tantalum barrier and the copper seed are preferably deposited by DC magnetron sputtering because of its speed and relatively low cost of equipment and source materials. However, sputter deposition into the deep and narrow via holes, that is, having a high aspect ratio, is difficult because sputtering is fundamentally a ballistic process with a nearly isotropic angular distribution pattern of sputtered ions. not immediately amenable to coating the sidewalls or even bottom of high aspect-ratio holes. This difficulty has in large part addressed by assuring that a large fraction of the sputtered atoms are ionized and then applying a strong electrical bias to the wafer so that the sputter ions are accelerated towards the wafer in much more vertical angular distribution and penetrate deep within the vias. The uniformity of these ionized sputter processes, particularly when applied to 300 mm wafer in a reasonably sized sputter chamber, has presented further challenges. Increasingly, auxiliary magnets are placed along the sidewalls of the sputter chamber to confine and guide the sputter ions.
Commercial sputter equipment needs to be precise, easy to align, and relatively inexpensive to fabricate and operate.